If you did a test recording on the same SD card, i.e., stopped, and played and restarted, unless you reformatted, the second recording should have shown up as *001.wav. The fact you only have a *000.wav file suggests that it wasn't stopped and restarted. (The Zoom manual actually says numbering starts with 0001!)

And, if there were two mics, why was it set to MONO mode? It makes no sense. (Actually, one possibility it suggests is that MONO mode was selected and the left, external mic level turned down to zero, or perhaps the mic turned off, because that's the only input recorded in that mode, per the manual.)

Same page shows that MONO prefix is used if "Mono Mix" is turned on. That is a silly mode that uses two inputs, blends them, and then sends the (mono) blend to both left and right channels of a stereo file. Basically -- records a stereo file with no stereo separation.

So MONO means either or both mics are mixed and sent to both L & R in the file. I stopped reading too soon. The presence of the file with nothing in it tells me the levels were zeroed or the mics turned off. Perhaps they weren't record enabled (not an H4n user)?

I mean, the obvious answer is that the main record button was turned on so the file created, but there was no input where the MONO mode expected/required it, so it recorded nothing.